Many different types of optical filters for selecting light of a particular wavelength have been proposed in conjunction with wavelength division multiplexed optical systems for use in optical communication systems.
Also the use of a single fixed diffraction grating for passive optical wavelength separation is well understood. The spectral components of the input source are separated and distributed around an output plane so that selection of the correct spatial region allows any spectral component to be isolated. Thus, spatial filtering of the output plane produces spectral filtering of the source. Such a technique can be used to demultiplex a wavelength division multiplexed signal consisting of a number of data-modulated wavelength channels provided that the size of the output devices are correctly matched to the data bandwidths.
A single fixed diffraction grating only separates the different wavelength channels and allows no recombination of these channels in the output plane. Re-selection of the output wavelengths i.e. somehow tuning such a filter can only be achieved by changing the grating, rotating it or somehow moving the spatial filtering device in the output plane. In the past these have been implemented by some form of servo-mechanical system with a feedback control to cause physical movement of the grating or of a photodetector or optical fibre receiving light in the output plane.
An example of an optical crossbar switch using a variable hologram to route light in a particular direction between its input and output and so, in effect, cause switching between a particular input of the crossbar switch and a particular output is described in an article entitled "A Holographicly Routed Optical Crossbar: Theory and Simulation" by D. C. O'Brien, W. A. Crossland and R. J. Mears published in Optical Computing and Processing, 1991, volume 1, number 3, pages 233-243.